All these people are making assumptions based on what happened in the past. The scene with him watching a military show makes him gay???
I honestly don't think Ricky knew at all, because his dad kept it so hidden, he became the opposite of the "typical gay', as a macho, military man. To him, you were either a strong, patriotic man, or you were a twink, no in between.
Ricky's response of "fags making him want to puke" was a perfect response to his father, knowing his extremist ideologies. To be merely tolerant, or indifferent of gays, was not an option.
In retrospect now based on the many examples we've seen in movies and real life, its the ones that have a huge problem with gays that are the closeted ones. But I personally don't think Ricky knows.
But Ricky's dad will definitely suspect that his dad killed Lester, based on their last conversation.

Ricky struck me as a meticulous person. He was a very good observer. In my opinion, Ricky told his dad that, "Those fags make me want to puke" not only to see if he would get a reaction from him, but to tell the Colonel's poker face what it wanted to hear. If Ricky is the mind reader I think he is (remember his response to his dad kicking him out? "You really mean that") it would have been obvious to him when the Colonel started to cry. I mean, if his dad wasn't a closeted homosexual and simply hated gay people, I think he would have beaten Ricky to death right then and there since he assaulted him before and said he'd rather let his son die than be a "fa99ot". In short, I believe he had his suspicions, but definitely knew the truth in their last scene together.

My impression was that Ricky said that gays made him sick in order to placate his father.
Ricky does this several times throughout the film, trying to keep his very unstable dad calm.
I'd say this cloud is Cumulo Nimbus.
Didn't he discover America?
Penfold, shush.

^ This. There's no other explanation for the delivery Ricky gives than this, since we find out later he was never unstable.
There's also the bit with the urine cup, which felt very uncomfortable, like there was some expectation he was going to do it right there.

I'm pretty sure Ricky had a very good speculation that his father was gay. In the scene where Ricky tells his father that gays make him sick I think it was to get a reaction out of him knowing deep down that it would hurt him to hear that from his own son.

Which scene are you referring to? I'm curious

nah. it was to show how
damaged
the mother was.
also, there are hints to the father being gay in the movie, like the television scene.

I seem to remember he said something to his mum about their marriage just before leaving and she said "wear a raincoat", which was strange but could have meant a condom [for whatever reason?]
http://www.kindleflippages.com/ablog/

It sounds like he suspected it.

I realize people don't usually like my answer, but it is true. Since these are fictional characters they don't "know" anything not specifically contained in the movie we see. I don't recall anything in the movie that would indicate to us that he knew his dad was closeted gay, and we did see (and hear) a number of reasons that his son would consider him a sad old man.
So my answer to your question is
"There is no way of knowing therefore it is moot."
.... TxMike ....
Take a risk, Take a chance, Make a change.
Kelly Clarkson - Breakaway